A forecast of 24 °C and a sun icon does not tell you whether the sea is for swimming. The sea lags behind the air by one or two months. June can feel like summer on the beach and like spring in the water. September is often the other way round: the air has softened, and the sea is the warmest it will be.
“Warm enough” is personal. As a working line for this guide, about 22 °C is a swim for most people, 20 °C is a swim if you are keen, and under 18 °C is a dip. Children and anyone who came for the water, not the challenge, should wait for the higher number.
These are typical conditions, rounded. A windy bay and a sheltered one on the same island can differ. Your week can differ from the month.
Month by month, in plain terms
April and May. Almost nowhere in Europe is a swimming holiday yet. The eastern Mediterranean and Cyprus are the first to reach the high teens. The Atlantic and the northern Med are cold. Go for walking and light, not for the water. The air side of this season is sunny destinations in spring.
June. The eastern Mediterranean, Cyprus and many Greek islands reach about 21–23 °C. A swim is reasonable if you are happy around 22 °C. Spain’s Mediterranean coast, Italy and Croatia are often still 19–22 °C: swimmable late in the month, chilly at the start. The Atlantic coast of Portugal and France is cooler than the air temperature suggests.
July. This is the first month the central and western Mediterranean is reliably a swimming sea, often 23–26 °C. Greece, Turkey and Cyprus are warmer. The Atlantic is still a different sea: Lisbon and the Algarve are often around 18–21 °C, which is why those beaches can be packed while people only paddle.
August. Peak for most of the Mediterranean, about 25–28 °C in the east and the south, a degree or two less in the north of the basin. The Black Sea around the Bulgarian coast is warm as well, often mid-20s. The Baltic is the exception people misread from the air temperature: 17–20 °C in a good year, and only for the hardy.
September. Often the best swimming month. The sea holds August’s heat, commonly 24–27 °C in the eastern and central Med, while the air drops out of the worst heat. West-facing Atlantic coasts stay cooler. If you want the water more than you want a beach club in peak season, this is the month.
October. The south and east of the Mediterranean are still around 21–24 °C, so Cyprus, Crete, much of Greece and the far south of Spain and Turkey still work. Further north, France and northern Italy have usually dropped under a comfortable swim. Storms become part of the month. See sunny destinations in autumn.
November to March. The Mediterranean is a walking sea, roughly 14–18 °C, warmer in the east than in the Gulf of Lion. Swimming is a choice, not the point of the trip.
The water that does not follow the Mediterranean
Canary Islands. The sea barely has a winter. It sits around 19–21 °C from December to February and around 22–23 °C in late summer. That is a year-round dip, and it is never a bath. People who want “warm water” in January are thinking of a different sea.
Red Sea. Hurghada and Sharm el-Sheikh stay swimmable through winter, typically the low 20s, and are properly warm from late spring into autumn. This is the winter beach if a swim is non-negotiable. How it compares with the Canaries and Agadir is in Canaries, Agadir or the Red Sea.
Agadir and the Atlantic coast of Morocco. Winter air can be 21 °C while the sea is still in the high teens. Do not book it as a swimming holiday and then blame the weather.
Baltic and North Sea. Summer air and long evenings are the attraction. The sea is a bonus in July and August if you like cool water. It is not a Mediterranean substitute, whatever the sunshine total says.
How to use this with a forecast
Sea temperature changes slowly, so the climate month is a better guide to the water than a 7-day air forecast. Use the forecast for the air, the wind and the rain. Use the month for the sea.
A sunny, calm week in early June on a Spanish beach can be a wonderful trip and a disappointing swim. A breezy week in late September can be the opposite. If the swim is the reason for going, pick the month first, then the forecast, then the journey. Beach or city break is the same distinction for the air.
Look for a beach warm enough to matter. The beach filter is deliberately strict about temperature. A beautiful coast in the mid-teens will not be offered as a beach.